XIV. W. H. WARREN. 
subject in regard to the data which it is sought to obtain in the 
tests, and the precautions to be taken in order to eliminate dis- 
turbing influences. It is also proposed to endeavour to establish 
the sizes and proportions of test pieces which are most suitable 
for the particular test in question, with reference to the data 
which it is the object of the test to furnish ; and, moreover, to 
secure uniformity in the results, and render their comparisons 
with those obtained in other countries much more satisfactory 
than at present. 
TENSION. 
Tensile tests are by far the most general in connection with 
the commercial testing of construction materials, and, when they 
are correctly made, furnish reliable data for the determination of 
the quality in regard to the suitability or otherwise of the material 
in guestion. It is essential that the following points be accurately 
determined :— 
a. The tensile strength per unit of area. 
b. The yield point. 
c. The ductility as measured by the percentage of elongation 
or construction of area after rupture. 
For accurate scientific testing, the limit, and coefficient of 
elasticity should be also determined, as well as the distribution of 
the elongation over the tested length in order that the general 
extension may be separated from the local, and that the elongation 
may be corrected when fracture occurs on one side of the centre 
of the tested length between the reference points. When the 
fracture does not occur in the centre the elongation is doubled for 
a distance equal to half the length of the test, counting from the 
section of rupture on the side, where it is possible to measure it, 
in such a manner as to render the same conditions as would have 
obtained had the rupture occurred in the middle of the test piece, 
and had the elongation been produced freely on both sides. This 
result increases slightly the effective elongation measured directly 
on the real test piece. The method is rather troublesome as the 
test piece should be marked before testing in }” or }” spaces 
